Date of Award

8-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical and Biomolecular Engineering

Committee Member

Dr. Amod A. Ogale, Committee Chair

Committee Member

Dr. Douglas Hirt

Committee Member

Dr. Christopher L. Cox

Abstract

The understanding of precursor flow profiles during melt spinning is a step towards producing desirable carbon fibers for structural applications from mesophase pitch. During the melt spinning process, flow during extrusion determines the cross-sectional fiber microstructure, which is crucial to carbon fiber strength. The subsequent fiber draw down is not known to alter the microstructure within the cross section. Also, prior modeling studies have varied fluid complexity but have not examined the details of spinneret geometry, such as a filter in the counterbore and capillary placement. Therefore, this study aimed to investigate fluid behavior during the extrusion component of melt spinning, through geometrically complex spinnerets. Modeling was conducted using finite element analysis (FEA) software package, ANSYS, version 17.0. The geometries and meshes were constructed with the Design Modeler module, whereas material and boundary conditions were established on the Polyflow solver. This study was initiated by validating the modeling protocol with prior literature results [Kundu and Ogale 2006] on AR-HP mesophase pitch rheology data on the ACER rheometer. Good agreement was observed between ANSYS and experimental viscosities, with a 7-14% difference in a Newtonian viscosity and a 0.1 - 5% difference in fitted Power Law models. For complex spinneret geometries, the Newtonian model was used to represent the fluid, since it approximates the viscosity of mesophase pitch under steady state conditions. The geometry graduated to modeling batch melt spinning equipment, comprised of a barrel/plunger assembly and a spinneret, consisting of a counterbore and capillary, and was examined across various barrel diameters. This comparison assessed the impact that the degree of transition from barrel to counterbore has on resulting flow fields and profiles. Smoother transition barrel to counterbore led to smaller vortex formations, as well as enhancing computational accuracy. With the addition of the filter at the barrel exit, pressure drop from barrel to counterbore exit showed an approximately 30% increase. However, no visible impact was noted on capillary pressure drop. Also because of this additional contraction, vortices were formed at the upper corners of the counterbore. Since an overall good agreement between ANSYS and analytical predictions was observed, a more complex geometry was examined. Spinnerets with multiple off-center capillaries, with respect to the counterbore, was also modeled. This geometry was of interest since machining imprecision leads to counterbore-capillary eccentricity. Thus, simulations were conducted at various inter-capillary distances. Wider inter-capillary distances (i.e. wider distance from counter center) resulted in more pronounced flow division, leading to larger area of vortex formation.

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